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Show 136 PROF. HUXLEY ON THE OVIDUCTS OF OSMERUS. [Mar. 20, middle, while below it is bounded by a smooth thin-edged band invested by peritoneum (fig. 1, B ) . A transverse section shows that the ovigerous lamellse pass under this reflected band (to which their outer edges are attached) to the ventral wall of tbe ovary. The groove inclosed by the reflected band is thus divided into a series of interlamellar loculi (fig. 1, C). Thus far the ovary agrees in all the essentials of its structure with that of the other Salmonidae, and with that of all adult Ganoids except Lepidosteus. Even in the latter, Balfour has shown that the ovary passes through a similar condition in the embryonic state. The mesoarium, however, does not stop at the posterior end of the ovary, but, as Rathke points out, the fold of peritoneum which constitutes it is continued backwards to the oviducal aperture; while laterally it passes into the peritoneal lining of the lateral walls of the abdomen, ending in a free concave edge immediately behind and on the outer side of the posterior extremity of the ovary. It thus forms the ventral boundary of a passage which opens in front by a wide ostium into the abdominal cavity (fig. 1, od. I, od.r). As the posterior end of the right ovary lies very far behind the posterior end of the left ovary, it follows that the right ostium is equally far behind the left, and that the right passage is by so much shorter than the left. The mesentery terminates by a free posteriorly concave edge (which contains the rectal artery) just opposite the level of the posterior end of the right ovary; and, behind this free concave edge of the mesentery, the right and left passages unite in a short but wide common chamber, which opens externally in the middle line, behind the anus and in front of the urinary outlet (fig. 1, g). In a Smelt ready to spawn, these passages, as well as the common chamber, are crammed full of ova; and it is obvious that, whatever their morphological nature, they are, in a physiological sense, oviducts, comparable to Fallopian tubes. But every one who is familiar with the anatomy of the female reproductive organs of the Ganoids, will at once perceive that these passages are the homologues of the oviducts of Acijwnser, Polyodon, Polypterus, and Amia (fig. 2, p. 137). Neither in structure, nor in their essential anatomical relations, is there any difference between them. It is true that, in tbe Ganoids in question, the oviducts com-municate with the renal ducts, and that tbe excretory aperture is common to the urinary and the genital apparatus, while in the Smelt there is no such communication and the oviducal and renal apertures are separate. But, among the Sturiones and in Lepidosteus, the renal are much wider than the genital ducts, and the communication between the two is effected far in front of the external aperture, while in Polypterus and Amia the oviducts are much wider than the ureters and the communication takes place near the external aperture. Thus the arrangement in Osmerus represents simply the third term of a series of modifications, tending towards the separation of the ureteric from the oviducal ducts, two terms of which are presented by the Ganoids. And it follows that the arrangement of the parts which obtains in the ordinary Salmonidae is a fourth term in |